1. Field of the Invention
This invention relates to a coated substrate, in particular to a coated transparent sheet providing a laminated assembly with a high selectivity, i.e. a high ratio of luminous transmission to energy transmission.
2. Description of the Related Art
Laminated assemblies comprising coated substrate sheets which provide the assemblies with high selectivity have become much used for vehicle windows, especially for motor cars and railway carriages. These duties pose the conflicting needs of providing adequate light transmission, in many instances as defined by legal regulations, while protecting the vehicle occupants against solar radiation. Desirably the window also presents a pleasing tint to the vehicle occupants and passers-by.
Several of the terms used for the properties of a coated substrate have precise meanings defined by an appropriate standard. Those used herein include the following, most of them as defined by the International Commission on Illumination--Commission Internationale de l'Eclairage ("CIE").
In the present specification, two standard illuminants are used: Illuminant C and Illuminant A, as defined by CIE. Illuminant C represents average daylight having a colour temperature of 6700K. Illuminant A represents the radiation of a Planck radiator at a temperature of about 2856K. This illuminant represents the light emitted by car headlamps and is essentially used to evaluate the optical properties of glazing panels for motor vehicles.
The term "luminous transmission" (TLA) used herein is as defined by CIE, namely the luminous flux transmitted through a substrate as a percentage of the incident luminous flux of Illuminant A.
The term "energy transmission" (TE) used herein is as defined by CIE, namely the total energy directly transmitted through a substrate without change in wavelength. It excludes the absorbed energy (AE), i.e. the energy which is absorbed by the substrate.
The term "selectivity" (SE) used herein is the ratio of luminous transmission (TLA) to energy transmission (TE).
The term "colour purity" used herein refers to the excitation purity measured with illuminant C as defined in the CIE International Lighting Vocabulary, 1987, pages 87 and 89. The purity is specified according to a linear scale on which a defined white light source has a purity of zero and the pure colour has a purity of 100%. For vehicle windows the purity of the coated substrate is measured from the side which is to form the external surface of the window.
The dominant wavelength (.gamma..sub.D) is the peak wavelength in the range transmitted or reflected by the coated substrate.
The terms "refractive index" and "spectral absorption index" are defined in the CIE International Lighting Vocabulary, 1987, pages 127, 138 and 139.
The substrate is most typically a vitreous material such as glass but can be another transparent rigid material such as polycarbonate or polymethyl methacrylate.
For various reasons, many of them related to considerations of sound or heat transmission or to safety in the event of breakage, the assembly normally comprises two or more laminated sheets. A typical laminated assembly comprises, in sequence, a first layer of glass, a layer of transparent adhesive such as polyvinylbutyral (PVB) and a second layer of glass. The thickness of each glass layer is typically in the range of 1.6 to 3 mm. The mean refractive index of the assembly, ignoring the effect of the coating layers, is typically 1.5. The coating is generally applied to the inner face (i.e. the face in contact with the adhesive) of the sheet that in use will form the external sheet of the assembly, but it can alternatively be applied to the inner face of the sheet that in use will form the internal sheet of the assembly.
A laminated assembly tends to have different optical properties from those of a single glass sheet. The differences arise mainly from the use of multiple sheets. Thus the properties required of, and achieved by, a laminated assembly differ from those of a single glass sheet. Care must therefore be taken in manufacturing a laminated assembly to make appropriate selection of the respective materials, thicknesses and coatings so as to ensure that the sought-after properties are achieved.
For road vehicle windows the legal requirement for luminous transmission (TLA) of windshields is at least 70% in USA and at least 75% in Europe. With regard to solar radiation, the total energy directly transmitted (TE) is desirably well below 50%. A further factor is the colour tint of the coated substrate, which should present a pleasing appearance. A pink tint may be found attractive and a green tint is even more so, which poses an additional problem in achieving the desired colour from the coating while retaining the required high luminous transmission and low energy transmission.
The requirements for railway carriage windows are similar to the above while not in every case so closely regulated by law. The need generally remains to keep the luminous transmission high and the energy transmission low.
For vehicle applications the purity of the reflected colour is preferably low. This has been found to be particularly difficult to achieve simultaneously with a high level of luminous transmission and a low level of energy transmission.
It has become increasingly popular to apply to glass sheets several coating layers, known as a stack, in order to modify their transmission and reflecting properties. Prior proposals have been made for metal and metal oxide layers in many different combinations to serve as the coating stack to impart selected properties to the glass. One recent combination of layers attracting attention has been the so-called "five layer" stack, typically comprising three layers of metal oxide applied alternately with two layers of metal.
U.S. Pat. No. 4,965,121 relates to such a stack for vehicle windshield glass and comprising in sequence from the substrate: a first layer, of dielectric material; a second layer, of partially reflective metallic material; a third layer of dielectric material; a fourth layer, of partially reflective metallic material; and a fifth layer, of dielectric material. The dielectric material is required to have a refractive index of 1.7 to 2.7. The first and fifth layers have substantially the same optical thickness but are 33-45% of the optical thickness of the third layer. The second and fourth layers have thicknesses within the range of 75-100% of each other. The claimed stack typically gives a high light transmission and a substantially neutral reflective visible light colour.
French Patent specification 2708926-A1 similarly relates to a five-layer stack, in this case to impart to vehicle or building glass a combination of high selectivity, i.e. a ratio of luminous transmission to energy transmission as high as possible, while retaining a pleasant visual aspect in reflection. It seeks to achieve this purpose by a stack comprising in sequence from the substrate: a first layer of dielectric material; a first metallic layer with infra-red reflecting properties; a second layer of dielectric material; a second metallic layer with infra-red reflecting properties; and a third layer of dielectric material. The first infra-red reflecting layer has a thickness of 55-57% of the second infra-red reflecting layer.